In recent years, GaN compound semiconductor materials, which are nitride semiconductors, have drawn attention as semiconductor materials for short wavelength light-emitting devices. The GaN compound semiconductor is formed on various kinds of substrates, such as a sapphire single crystal substrate, an oxide substrate, and a III-V group compound substrate, by a metal organic chemical vapor deposition (MOCVD) method or a molecular beam epitaxy (MBE) method.
The gallium nitride compound semiconductor device is characterized in that a small amount of current is diffused in the horizontal direction. Therefore, a current is applied to only a semiconductor immediately below an electrode, and light emitted from a light-emitting layer is shielded by the electrode, which makes it difficult to emit light from the light-emitting device to the outside. In addition, in the light-emitting device, a translucent positive electrode is generally used, and light is emitted to the outside through the translucent positive electrode.
The translucent positive electrode is formed of a known conductive material, such as Ni/Au or ITO. However, metallic materials, such as Ni/Au, have a low contact resistance with a p-type semiconductor layer, but have a low light transmittance. On the other hand, oxides, such as ITO, have a high light transmittance, but have a high contact resistance.
A method of performing annealing in a nitrogen (N2) or hydrogen atmosphere has been proposed in order to reduce the specific resistance of a conductive oxide film, such as an ITO film. The annealing process increases oxygen vacancies in the conductive oxide film and the carrier density of the conductive oxide film. The increase in the carrier density causes the specific resistance of the conductive oxide film to be reduced. In general, the reduction action of the oxide film is stronger in a gas atmosphere including hydrogen than in a gas atmosphere including nitrogen. Therefore, when the annealing process is performed in a gas atmosphere including hydrogen, it is possible to obtain a conductive oxide film having low specific resistance.
However, in the annealing process performed in the gas atmosphere including hydrogen, hydrogen is bonded to Mg or Zn doped into a p-type semiconductor layer of a gallium nitride compound semiconductor light-emitting device, and hinders impurities, such as Mg or Zn, from serving as acceptors. The bonding between the acceptor impurities and hydrogen causes an increase in the specific resistance of the p-type semiconductor layer.    [Patent Document 1] JP-A-6-88973    [Patent Document 2] Japanese Patent No. 2540791